If co-instructors gave different syllabi with different grading schemes, is this generally considered a valid reason to challenge grade?

The majority of the question's text concerns minutiae about syllabi and percentages that are unlikely to be of interest to anyone except the OP. The question would be significantly improved by eliding / summarizing this information and concentrating on the core question:

What happens when multiple, conflicting syllabi are given out for the same course? Does the student have a legitimate grievance when one syllabus is being followed but not the other, at the expense of his grade?

I think these are reasonable questions for our site.

My answer to the first question is: yikes. Three different instructors for the same course, three different syllabi, and one of the instructors (the chair!) does not even know about both of the other syllabi? What a mess. In general having courses taught by more than one faculty member makes things harder and necessitates much more explicit, advanced planning. This course does not sound well-planned. As soon as you get talk about which syllabus is more "official" than the others, things are not going well. I don't think there is a standard, easy answer to this question, and I don't see any coherent answer which does not admit mistakes on the part of the faculty. On the other hand I wonder why this issue did not come up earlier in the course: on the first day of class every student was in possession of two contradictory syllabi and no one noticed? I find that curious and am not sure what to make of it.

My answer to the second question is: yes, I think so. If you show up to some higher administrative official with two syllabi in hand and say that you want to be held to the standard of the syllabus you were given first, you have a good shot at getting some traction on this. However I would encourage you to try to resolve the matter as non-combatively as possible: if you are a graduate student in the program, getting the chair chewed out by (say) her dean is likely to have some effect on your course grade, but it may have other, less pleasant effects down the line. I would begin by doing what you have already done: clearly pointing out the discrepancy between the two syllabi and the significant effect it may/will have on your course grade. You should indicate that you proceeded in the course with an understanding based on the first syllabus that you received and that you would like this to be taken into account in the course grade. Give them a chance to do this for you before you escalate the situation.

Finally, the business about the 75 belonging to two grade ranges: in my opinion you look petty by bringing that up at all (look at the reaction your question has received). The purpose of the grading system being spelled out in advance is for students to be sure they are treated fairly and to be able to have some rough means of predicting what kind of performance will lead to what kind of grade. There is nothing that you or any student would have done differently if 75 belongs to one grade range rather than the other. I agree with other commenters who said that the time you spent complaining about that would be better spent learning the material and improving your performance in the course. Mixing legitimate, serious grading concerns with "grade-grubbing" is not a good strategy.